This disclosure relates generally to the field of directional antennas for transmitting and/or receiving electromagnetic radiation, particularly (but not exclusively) microwave and millimeter wavelength radiation. More specifically, the disclosure relates to antennas with serial feed that transmit and/or receive a directionally shaped and steered electromagnetic beam that is formed along the path of the propagating in the feed electromagnetic signal. These antennas, commonly referred to as scanning antennas, are well-known in the art, as exemplified by U.S. Pat. Nos. 6,750,827; 6,211,836; 5,815,124; and 5,959,589, the disclosures of which are incorporated herein by reference. One class of these antennas, which may be termed dielectric waveguide fed antennas, operate in the transmit mode by the evanescent coupling of electromagnetic waves traveling in an elongate (typically rod-like) dielectric waveguide (or “feed line”) to a scanning antenna element (typically, a rotating cylinder or drum), and then radiating the coupled electromagnetic energy in directions determined by surface features of the antenna element. Conversely, in the receive mode, the electromagnetic energy received from the free space by the antenna element is coupled into and travels in the dielectric waveguide. By defining rows of scattering features, wherein the features of each row have a different period, and by rotating the antenna element around an axis that is parallel to that of the waveguide, the radiation can be directed in a plane over an angular range determined by the different periods, thereby transmitting and/or receiving a highly directional beam with a desired beam shape.
In the context of this disclosure, the term “beam shape” encompasses the beam direction, which is defined by (a) the angular location of the power peak of the transmitted/received beam with respect to at least one given axis, (b) the beam width of the power peak, and (c) the side lobe distribution of the beam power curve.
Serial-feed scanning antennas are typically restricted to the first negative order of radiating space harmonics for transforming the guided electromagnetic signal energy to a single shaped beam propagating in free space with a given set of beam shape parameters and in a given direction. The scanning ability of such antennas is thus limited to the “negative” half space, meaning, generally, the angular portion of scanning range between the signal input to the waveguide and 0°, thus excluding the “positive” half space, meaning, generally, the angular portion of the scanning range between 0° and the end of the waveguide connected to an impedance-matching load. The scanning range, in fact, also typically excludes the zero-degree direction from the beam forming/scanning due to high constructive return interference in the “stop band” near the 0° scanning angle, and the low radiation efficiency commonly associated with such antennas.
It would therefore be an advance in the field of scanning antennas to provide a serial feed antenna that addresses the above-noted problem without undue complexity and in a cost-efficient manner. In particular, it would be advantageous to provide such an antenna with the ability to allow beam scanning in both halves (i.e., “negative” and “positive”) of the scanning space.